1. Field of the Invention
This invention relates generally to a method for detecting trace gases in the air and, more particularly, to a method of radiating a chemical cloud to heat the cloud and increase its temperature relative to the background, and then detecting chemicals in the cloud by spectroscopy.
2. Discussion of the Related Art
It is known in the art to detect certain constituents in a chemical cloud in the air by spectral analysis of the molecules making up the cloud. This type of chemical detection has many applications, including detecting natural gas leaks from underground pipes, chemical clouds from chemical spills, volatile organic vapor (VOC) from chemical processes, pollution from smoke stacks and the like, military chemical warfare agents, and other toxic gases present in the air. Typically, this type of spectral analysis of a chemical cloud is performed remotely, sometimes up to 10-20 km away, because the constituents in the cloud may be toxic, and thus a threat to health, or it may not be possible to directly detect the chemical cloud. The distance the detecting instrument has to be from the cloud for this remote type of passive sensing depends on the particular application, and different systems exist for different applications.
To perform this type of detection and analysis, a spectrometer, such as a Fourier transform infrared (FTIR) spectrometer, is directed towards the chemical agent cloud from a remote location, so that it passively receives emissions therefrom. If the chemical cloud is warmer than the background, such as sky, mountains, or other terrain, along the field-of-view of the spectrometer, target molecules in the cloud will exhibit emissions having an energy greater than the background emissions from the sky. If the chemical cloud is the same temperature as the sky, the target molecules within the cloud are absorbing photons at the same rate that they are emitting photons, so that there is no net energy exchange between the cloud and the background, and no difference relative to the background. As the temperature of the cloud increases, more photons are released from the chemicals in the cloud, which are available to be received by the spectrometer.
The spectral display generated by the spectrometer from the emissions provides emission lines and bands at certain wavelengths that is indicative of the atoms and molecules in the cloud. Because each material has its own spectral xe2x80x9cfingerprintxe2x80x9d representative of its molecules, the detected spectral display can be compared to a known xe2x80x9cfingerprintxe2x80x9d of a particular chemical to determine if that chemical exists in the cloud.
A problem exists with the passive remote sensing techniques that are currently used in the art because the temperature difference between the chemical cloud and the sky is often very small. In many cases, the temperature of the cloud is only about 2-3xc2x0 C. warmer than the temperature of the background. Because of such a small temperature difference, the detectable emissions from the cloud is typically very weak. This results in a poor signal-to-noise ratio, and thus poor detection sensitivity and possibly a high false alarm rate.
What is needed is a remote chemical detection system that causes the chemical cloud to be heated so that the temperature of the cloud is significantly different than the background. It is therefore an object of the present invention to provide a remote chemical detection system of this type.
In accordance with the teachings of the present invention, a technique for the remote detection and analysis of trace chemicals in the air is disclosed. A beam of electromagnetic radiation from an electromagnetic radiation source is used to radiate a suspected chemical cloud. The radiation energy that is absorbed by the cloud is quickly thermalized due to a rapid collision energy transfer between the molecules that absorb the radiation and the surrounding air molecules. This collisional energy redistribution will result in heating the chemicals in the cloud. An increase in the temperature of the cloud will increase the emission intensity of the molecules against the background, resulting in an improvement in the detection of the chemicals.
A tracking telescope is then used to collect the thermal emissions of the target molecules generated by the radiation. The tracking telescope can be located in the vicinity of the electromagnetic radiation source such that the viewing axis of the telescope is preferably coaxial with the propagation direction of the electromagnetic radiation. A spectrometer, such as an FTIR spectrometer, is used to resolve the emissions from the cloud that are enhanced by the radiation and to generate an emissions spectrum. The emissions spectrum is used to identify suspect molecules in the cloud by comparing the detected emissions to the known xe2x80x9cfingerprintxe2x80x9d vibrational spectrum of the suspect molecules.
The electromagnetic radiation source, telescope and FTIR spectrometer can be housed on a platform to scan over a wide area for surveillance of chemical clouds. Alternatively, the tracking telescope and the FTIR spectrometer can be located at a separate location from that of the electromagnetic radiation source. The viewing axis of the telescope will intersect with the beam of the electromagnetic beam. In this arrangement the location of the chemical cloud can be determined based on the intersection of the electromagnetic radiation beam and the view axis of the telescope.
The electromagnetic radiation can be microwave, millimeter wave, infrared, visible, or ultraviolet radiation. The wavelength of the electromagnetic radiation can be selected to be in resonance with the absorption lines of the chemicals, or of water vapor or oxygen molecules that are commonly present in the cloud. If the wavelength of the electromagnetic radiation is chosen to be in resonance with the absorption lines of the target chemical molecules, the returned emission intensity, as a function of the excitation wavelength, can be used to provide an increased discrimination of the chemicals against possible interference background chemicals.
Additional objects, features and advantages of the present invention will become apparent from the following description and appended claims taken in conjunction with the accompanying drawings.